This invention relates to classifying apparatus for classifying particulate material according to the particle size.
As is well known in the art the capacity of classifying apparatus is limited by the volume of a cell. More particularly, a high efficiency of classification (the percentage of the particles collected having desired particle size) is essential to classifying apparatus. Although it is possible to maintain the desired efficiency up to a predetermined quantity of the material to be classified commensurate with the capacity of the apparatus, when the quantity of the material loaded in the apparatus exceeds its capacity a large quantity of coarse particles would be accompanied by fine particles and a large quantity of fine particles would be accompanied by coarse particles thus making it difficult to perform a desired classification. For this reason, it is necessary to increase the size or capacity of the apparatus for classifying a large quantity of the particulate material. However, in a cyclone type classifying apparatus wherein a whirling flow of air or water in a cell is utilized to effect classification, increase in the volume of the cell results in only a small increase of the classifying capacity. Thus, even when the volume of the cell is doubled the classifying capacity would never be doubled, but increased only 20 to 30% at the best. Where classification is effected by a whirling flow as in the cyclone type apparatus, increase in the volume of the cell results in only a small increase in the stroke (which acts as the classification zone) of the whirling flow formed along the inner surface of the cell. Rather the volume in which the fluid stays would be increased. As the volume of the cell of the classifying apparatus increases, the quantity of air or other fluid to be blown in should also be increased proportionally. It is also necessary to increase the size of the dispersion plates and whirling vanes provided in the cell for effecting classification thereby increasing the costs of installation and running. Generally speaking, increase in the volume of the cell does not result in proportional increase in the classifying capacity.
In the operation of classifying apparatus utilizing upward whirling flow of a mixture of fluid and particulate material, until the concentration of the particulate material reaches a definite value separation of coarse particles is not satisfactory. It is presumed that the falling of coarse particles is prevented by the upward whirling flow so that while the concentration of the particulate material is low, the downward flow of separated coarse particles is disturbed by the upward whirling flow.